The present invention is directed to an electrolyte system for a metal-air battery having high capacity and recycle efficiency.
Lithium ion technology has dominated the market as energy source for small electronic devices and even hybrid electric vehicles. However, Li-ion batteries have insufficient theoretical capacity to be a power source for future high capacity generations of power sources capable to run an electric vehicle.
Metal-air batteries have been under investigation as advanced generation of high capacity energy sources that have the potential to power vehicular devices for distances comparable to present hydrocarbon based combustion engines. In a metal-air battery, the metal of the anode is oxidized and the resulting cation travels to the cathode zone containing a porous matrix of a material such as carbon, for example, where oxygen is reduced and the reduction product as oxide or peroxide combines with the metal cation to form the discharge product. Upon charge, this process is ideally reversed. Metal-air batteries are recognized to have potential advantageous properties over metal ion batteries because the cathodic material, oxygen, may be obtained from the environmental air atmosphere and thus the capacity of the battery would in theory be limited by the anodic metal supply. Thus, oxygen gas would be supplied continuously from outside the battery and battery capacity and voltage would be dependent upon the oxygen reducing properties and chemical nature of the discharge product formed.
Metal-air batteries typically include a metal electrode at which a metal is oxidized, an air electrode at which oxygen is reduced, and an electrolyte for providing ion conductivity. A significant limiting factor with conventional metal-air batteries is the evaporation of the electrolyte solution (i.e., the ionically conductive medium), particularly the evaporation of the solvent, such as water in an aqueous electrolyte solution and organic solvent in a non-aqueous electrolyte solution. Because the air electrode is required to be air permeable to absorb oxygen, it also may permit the solvent vapor to escape from the cell. Over time, the cell's operation efficiency is reduced due to solvent depletion.
Other problems associated with aqueous electrolyte batteries include water electrolysis during recharging and self discharge. During recharge, a current is passed through the battery to reduce the oxidized fuel at the fuel electrode. Some of the current, however, electrolyzes the water resulting in hydrogen evolution (reduction) at the fuel electrode and oxygen evolution (oxidation) at the oxygen electrode as represented in the following equations:2H2O(l)+2e−→H2(g)+2OH−(aq)  Reduction:2H2O(l)→O2(g)+4H+(aq)+4e−  Oxidation:In this manner, further aqueous electrolyte is lost from the battery. Additionally, the electrons that are consumed in reducing hydrogen are not available to reduce the oxide. Therefore, the parasitic electrolysis of the aqueous electrolyte reduces the long term cycle efficiency of the secondary battery.
To compensate for these problems, metal-air batteries with aqueous electrolyte solutions are typically designed to contain a relatively high volume of electrolyte solution. Some cell designs even incorporate means for replenishing the electrolyte from an adjacent reservoir to maintain the electrolyte level. However, either approach adds to both the overall size of the cell, as well as the weight of the cell, without enhancing the cell performance (except to ensure that there is a significant volume of electrolyte solution to offset evaporation of the water or other solvent over time). Specifically, the cell performance is generally determined by the fuel characteristics, the electrode characteristics, the electrolyte characteristics, and the amount of electrode surface area available for reactions to take place. But the volume of electrolyte solution in the cell generally does not have a significant beneficial effect on cell performance, and thus generally only detracts from cell performance in terms of volumetric and weight based ratios (power to volume or weight, and energy to volume or weight). Also, an excessive volume of electrolyte may create a higher amount of spacing between the electrodes, which may increase ohmic resistance and detract from performance.
Metals employed as oxidizable anode materials include any metal, alloy or metal hydrides thereof. For example, the fuel may comprise transition metals, alkali metal, and alkaline earth metals. Transition metals include, but are not limited to zinc, iron, manganese, and vanadium. The most common alkali metal is lithium but other alkali metals including sodium may be used. The other metals include, but are not limited to magnesium, aluminum, calcium and gallium. The metal electrode may comprise a metal, including elemental metal, metal bonded in a molecule or complex, including oxides, metal alloys, metal hydrides, etc.
The metal electrode may have any construction or configuration and may be a porous structure with a three-dimensional network of pores, a mesh screen, a plurality of mesh screens isolated from one another, or any other suitable electrode. The fuel electrode includes a current collector, which may be a separate element, or the body on which the fuel is received may be electroconductive and thus also be the current collector.
Lithium air batteries have the potential to supply 5-10 times greater energy density than conventional lithium ion batteries and have attracted much interest and development attention as a post lithium ion battery technology. For example, a nonaqueous lithium air battery which forms Li2O2 as discharge product theoretically would provide 3038 Wh/kg in comparison to 600 Wh/kg for a lithium ion battery having a cathodic product of Li0.5CoO2. However, in practice, the metal air technology in general and specifically current nonaqueous lithium air batteries suffer many technical problems which have prevented achievement of the theoretical capacity.
The capacity of the Li air battery is highly dependent upon the capacity of the cathode matrix to store the Li2O2 discharge product. Li2O2 is generally insoluble in conventional nonaqueous solvents employed in metal air batteries. Therefore, upon formation at the cathode matrix the Li2O2 precipitates and fills the surface porosity of the matrix thus preventing access to the vacant capacity of the matrix interior region. Moreover, Li2O2 is an insulator and therefore, once the surface of the matrix is coated, oxygen reduction is prevented and discharge terminated, i.e., the capacity of the battery is severely reduced in comparison to the theoretical capacity.
Furthermore, the cathode performance is strongly affected by the moisture content of ambient air. To simplify the cathode reaction mechanism, much effort has been devoted to battery systems having a supply of pure oxygen to the cathode. However, practically speaking, the structure, cost and equipment necessary for such a system detracts from the potential advantages. To be of utmost utility the metal air battery will require utility of ambient air.
In non-aqueous Li-air battery, water is detrimental to battery performance as described above. To date in spite of much experimental effort and study, no practically feasible method to develop a metal-air battery that functions efficiently with ambient air as oxygen source has been developed. One consideration may be to dry the ambient air in advance before introduction into the battery. However, in order to decrease the water content of air to an acceptable level (less than hundreds ppm), the dehydration system required would be too large. This will be also unrealistic for the installment of Li-air battery.
In view of the problems associated with non-aqueous electrolyte metal-air batteries, selecting an aqueous Li-air battery system may be considered. However, in an aqueous system, a highly concentrated alkaline solution is formed about the cathode which is corrosive to surrounding materials of construction. Also, in an aqueous system, water functions both as an electrolyte solvent and an active material. As a consequence the water content of the system is depleted during operation of the battery and requires a certain level of humidity in the environmental air supply to remain functional. However, this is not feasible for batteries operating in an environment of elevated or freezing temperatures. Thus water management is an element which is key to success of the aqueous Li-air battery.
The purpose of this invention is to develop new electrolyte solvents containing water that may have general utility for metal-air batteries and specifically for Li-air batteries. This novel electrolyte will be suitable for utility as a non-aqueous electrolyte solvent as well as aqueous electrolyte solvent.
Effort to overcome the problems listed above for metal-air batteries and to produce an efficient high capacity metal-air battery has received much attention.
Best et al. (U.S. 2014/0125292) describes lithium ion or lithium metal batteries containing electrolyte systems containing ionic liquids. A water content of less than 1000 ppm in the ionic liquid is described as allowable. However, lower levels, “less than 750 ppm, less than 500, less than 250 ppm, . . . ” are described as preferred embodiments. Best does not describe a two phase aqueous/ionic liquid composition. It is believed that the water levels described in this reference are dissolved amounts and therefore a single phase electrolyte solvent is disclosed.
Friesen et al. (U.S. Pat. No. 8,481,207) describes a metal air battery containing an ionic liquid as an electrolyte component and an anode constructed of a porous material capable to absorb a liquid metal upon charging. By use of a liquid anode metal dendrite formation is avoided thus preventing shorting of the battery due to dendrite extension to the cathode. Friesen describes that the ionic liquid may contain as much as 100 ppm water. The water at 100 ppm would represent water dissolved or absorbed by the ionic liquid. Nowhere does this reference describe a two phase electrolyte system having both aqueous phase and ionic liquid phase.
Tsukamoto et al. (U.S. Pat. No. 6,797,437) describes a lithium ion secondary battery having an anode of lithium metal or porous material capable of absorbing and releasing lithium ions and a cathode of a complex oxide of lithium and a transition metal. The electrolyte is a soluble lithium salt in a two phase electrolyte system containing a carbonate and/or ether combination. The second phase is formed by a halogen containing flame retardant material. Tsukamoto does not disclose an ionic liquid as an electrolyte component and does not disclose or suggest a two phase electrolyte having aqueous and ionic liquid phases. Further, Tsukamoto does not describe a metal-air battery.
Parker et al. (U.S. Pat. No. 4,377,623) describes a zinc-halogen electrochemical cell having inert electrodes which support oxidation of zinc metal added to the cell and bromine added to the cell. The electrolyte liquid is a two phase combination of an aqueous phase containing halide ions and an organic nitrile phase containing halogen. Parker is not directed to a metal-air battery and does not disclose or suggest an ionic liquid as an electrolyte solvent.
Kuboki et al (Journal of Power Sources 146 (2005) 766-769) describes a study of hydrophobic ionic liquids for use in a lithium air battery. The equilibrium water content of several ionic liquids is determined by the amount of water absorbed from the atmosphere over an extended number of days. However, Kuboki reports that cell performance did not correlate with water absorption of the ionic liquid and also reports ionic liquids based on the imidazolium cation have small potential window of stability. Kuboki does not disclose or suggest a lithium-air battery containing a two phase electrolyte having an aqueous phase and an ionic liquid phase.
Xu et al. (Electrochemistry Communications 7 (2005) 1309-1317) describes a freestanding polymer gel electrolyte membrane for a zinc battery. The membrane is a lattice of poly(vinylidene fluoride-co-hexafluoropropylene) containing a zinc salt and a room temperature ionic liquid. Such membranes would be useful for construction of solid state batteries. Xu describes utility of the zinc ion conducting membrane in a zinc-air battery. This reference does not disclose or suggest a two phase electrolyte having an aqueous phase and an ionic liquid phase.
Friesen et al. (U.S. 2011/0305959) describes a metal-air battery having an ionic liquid based electrolyte. Metals suitable as anode materials include transition metals, alkali metals and alkali earth metals. Zinc air systems are most fully described. Friesen discusses the necessity to maintain a sufficient level of water in the electrolyte composition to drive the charge and discharge process and “tunes the ionic liquid” to contains water from 0.001 to 25% by adding a hydrophilic or hygroscopic additive which effectively absorbs water into the electrolyte. The amount of water absorbed depends on the nature and amount of the additive. Examples given for a zinc-air system include zinc chloride, zinc tetrafluoroborate zinc acetate and Zn(TFSI)2. Although Friesen discloses as much as 25% water, a 2 phase ionic liquid electrolyte system is never explicitly disclosed or suggested.
Zhang et al. (Chem. Commun., 2010, 46, 1661-1663) describes a lithium air battery employing an electrolyte system composition of acetic acid/water and lithium acetate. The lithium acetate is also indicated to be formed by oxidation of the lithium metal anode. Construction of a lithium-air electrochemical cell (Li/PEO18LiTFSI/LTAP/HOAc—H2O—LiOAc/Pt-carbon black) is described and an energy density estimated to be 1.478 W h/kg. Zhang does not disclose or suggest an ionic liquid as an electrolyte component or a two phase (aqueous/ionic liquid) electrolyte system.
Gasteiger et al. (Electrochemical and Solid State Letters, 15 (4) A45-A48 (2012)) describes a study of the effect CO2 and water on the performance of a lithium-air battery. The study showed that small amounts of water enhance capacity of the cell. Ionic liquids and/or a two phase aqueous/ionic liquid system is not disclosed or suggested.
Yuan et al. (Journal of the Electrochemical Society, 161 (4) A451-A457 (2014)) describes a study of the electrochemical performance of cells containing room temperature ionic liquids (RTIL) with as much as 1.0% water. The solubility of water in the same RTIL (1-butyl-1-methyl-pyrrilidinium bis (trifluoromethanesulfonyl)imide) (BMP-TFSI) was shown to be 1.1407 wt % and therefore, a two phase system is not disclosed. Additionally, Yuan does not disclose or suggest a metal-air battery with an electrolyte containing a room temperature ionic liquid (RTIL) water mixture.
In spite of the significant ongoing effort there remains a need to develop and produce an effective electrolyte system for a high capacity metal-air battery useful especially for powering vehicles to distances at least equal to or competitive with current hydrocarbon fuel systems.